Note [Quick Look overview]

The implementation of Quick Look closely follows the QL paper
   A quick look at impredicativity, Serrano et al, ICFP 2020
   https://www.microsoft.com/en-us/research/publication/a-quick-look-at-impredicativity/

All the moving parts are in this module, GHC.Tc.Gen.App, so named
because it deal with n-ary application.  The main workhorse is tcApp.

Some notes relative to the paper

(QL1) The "instantiation variables" of the paper are ordinary unification
  variables.  We keep track of which variables are instantiation variables
  by giving them a TcLevel of QLInstVar, which is like "infinity".

(QL2) When we learn what an instantiation variable must be, we simply unify
  it with that type; this is done in qlUnify, which is the function mgu_ql(t1,t2)
  of the paper.  This may fill in a (mutable) instantiation variable with
  a polytype.

(QL3) When QL is done, we turn the instantiation variables into ordinary unification
  variables, using qlZonkTcType.  This function fully zonks the type (thereby
  revealing all the polytypes), and updates any instantiation variables with
  ordinary unification variables.
  See Note [Instantiation variables are short lived].

(QL4) We cleverly avoid the quadratic cost of QL, alluded to in the paper.
  See Note [Quick Look at value arguments]